Biogasification of solid waste with an anaerobic-phased solids-digester system

ABSTRACT

The present invention provides methods for the generation of methane by a two phase anaerobic phase system (APS) digestion of organic substrates. Also provided is a device for practicing the methods of the invention. The APS-digester system is a space-efficient, high-rate solids digestion system. The APS-digester system consists of one or more hydrolysis reactors and one biogasification reactor.

FIELD OF THE INVENTION

[0001] This invention relates to improved two phase anaerobic digestionhaving separated hydrolysis and biogasification reactors which convertbiomass to desired methane product gas with high efficiency.

BACKGROUND OF THE INVENTION

[0002] Anaerobic digestion has been known to stabilize sludge and otherpredominantly organic materials, and usable product gas, of varyingcomposition, has been obtained from such anaerobic digestion processes.The organic feed mixture which provides the substrate for anaerobicbiodegradation can comprise a wide variety of organic carbon sources,ranging from raw sewage sludge to municipal refuse, or biomass materialsuch as plants and crop wastes. The process of anaerobic digestiondegrades any of these organic carbonaceous materials, under appropriateoperating conditions, to product gas which contains the desirablemethane gas.

[0003] Anaerobic digestion uses a consortium of natural bacteria todegrade and then convert an organic substrate into a mixture of carbondioxide and methane. The existing anaerobic digestion systems fororganic substrate digestion can be separated into two major types, onephase systems and two phase systems. Existing one phase systems includethe batch digester, completely mixed digester and the plug flowdigester. These one phase systems, in which the organic substrate andthe microorganisms are housed together are easy to operate and of lowcost. Completely mixed digesters and plug flow digesters requirecontinuous handling of feedstock and do not operate in batch mode.Further, the biogas produced in one phase systems consists primarily ofcarbon dioxide in the early stages of digestion. The high carbon dioxidecontent of the biogas is attributable to the slow growth of themethanogenic microorganisms and their inhibition by high concentrationsof volatile fatty acids (VFAs). In order to reduce the inhibition of themicroorganisms by the VFAs, the two phase digester has been introduced.

[0004] Separated two phase anaerobic digestion systems have been foundto enhance the conversion efficiency, such as described in Pohland andGhosh, Biotechnol. and Bio-eng. Symp. No. 2, 85-106 (1971), John Wileyand Sons, Inc. and by the same authors in Environmental Letters, 1:255-266 (1971). A typical two phase anaerobic digester system comprisesan acid phase digester and a biogasification reactor. The acid phasedigester is usually designed as a solid-bed batch reactor where solidwaste is housed and leached soluble compounds are collected. In the acidfirst phase, the microbial population and operating conditions areselected to promote the conversion of organic carbonaceous materials tocarbonaceous materials of lower molecular weight, primarily volatilefatty acids. The liquid and solid effluent from the acid phase isconveyed to a biogasification second phase, where methanogenic organismsconvert the volatile fatty acids to product gas that is composedprimarily of methane and carbon dioxide. Product gas is removed from thebiogasification reactor and processed, or scrubbed, to separate themethane component that is drawn off as pipeline gas.

[0005] Two phase anaerobic digestion has been carried out in a singlereactor as taught, for example, by U.S. Pat. No. 4,735,724 which teachesa non-mixed vertical tower anaerobic digester and anaerobic digestionprocess which provides passive concentration of biodegradable feedsolids and microorganisms in an upper portion of a continuous digestervolume and effluent withdrawal from the middle to the bottom portion ofthe digester, resulting in increased solids retention times, reducedhydraulic retention times and enhanced bioconversion efficiency.

[0006] U.S. Pat. No. 4,022,665 discloses certain specific operatingconditions for a two phase anaerobic digestion process, such as feedrates and detention times, which promote efficient conversion of organicmaterials. Additionally, the '665 patent discloses two separatedbiogasification reactors, a biogasification reactor I operated in serieswith a biogasification reactor II. The biogasification reactor IIreceives effluent fluid and/or effluent gas from biogasification reactorI. A somewhat similar process is disclosed in U.S. Pat. No. 4,696,746which teaches a process for two phase anaerobic digestion with twodiscrete biogasification reactors operated in parallel.

[0007] U.S. Pat. No. 3,383,309 teaches that the rate and efficiency ofthe anaerobic digestion process, particularly in the methane formingphase, are increased when hydrogen gas is introduced into the digestersludge. According to the '309 patent, hydrogen gas is introduced intoboth the acid forming and the methane forming phases, to increase theavailability of energy rich “hyper-sludge.” All improvements disclosedin U.S. Pat. Nos. 4,022,665, 4,696,746 3 and 383,309 can be adapted foruse according to the improved process of the present invention and theteachings of that patent are incorporated herein by reference.

[0008] French Patent No. 78 34240 describes an apparatus forbiogasification which is known in the art as an upflow sludge blanketreactor. This apparatus utilizes a two-stage digestion apparatus. Theapparatus is designed for and uses continuous recirculation between thereactors of the two stages. Continuous recirculation requires arelatively complex apparatus including filters, pumps and manifoldedinlets to disperse the recirculated liquid stream and to avoid itsAshort circuiting directly to the outlet of the reactor into which itwas just circulated. Additionally, the continuous recirculation requirestwo pumps that must operate continuously. In contrast, the presentinvention utilizes intermittent recirculation.

[0009] The sequential batch anaerobic composting (SEBAC) reactor is arelatively new digestion system. See, Chynoweth et al., Appl. Biochem.Biotech. 28: 421-32 (1991). The SEBAC system consists of three reactors.Each reactor operates as a single phase batch digester. The threereactors are interconnected and operated on a different digestionschedule, the first being newly started, the second running in themiddle of a digestion and the third running toward the end of adigestion. When new feedstock is loaded into the first reactor, theliquid from the third reactor is transferred to the first reactor toinoculate the feedstock and speed-up the digestion process.

[0010] A broad range of organic substrates are appropriate feedstocksfor biogasification reactors. An exemplary feedstock is agriculturalwaste. Agricultural waste consists mostly of carbonaceous organicmaterials and it presents a particularly attractive renewable source ofraw material for the generation of methane. The use of agriculturalwaste for this purpose serves a dual purpose, it produces a usefulproduct and reduces the volume of agricultural waste which must bedisposed of. Many different types of agricultural waste can be digestedutilizing a two phase anaerobic digestion scheme. The waste from theproduction of rice provides a salient example.

[0011] In California, for example, large quantities of rice straw areproduced each year as by-products of rice production. In the SacramentoValley alone, 1,452,000 tons of rice straw were produced in the cropyear of 1994-1995 (CARB-CDFA, Progress report on the phase down of ricestraw burning in the Sacramento Valley Air Basin, Report To TheLegislature, California Air Resources Board and California Department ofFood and Agriculture (1995)). Due to lack of feasible conversiontechnologies, however, utilization of these materials for energyproduction has not become practical for the agricultural sectors.

[0012] Current methods for disposal of these agricultural residuematerials have caused widespread public concerns with regard to theirenvironmental impact. In the case of rice and wheat straw disposal, forexample, open field burning is considered as a practice causing seriousair pollution problems, because of the emissions of smoke and other airpollutants, such as gases, particles and aerosols.

[0013] Current California legislation (the Connelly-Areias-Chandler RiceStraw Burning Reduction Act of 1991) mandates the rice growers to phasedown burning of rice straw, requiring a reduction in rice straw acreageburning to no more than 25% of the planted acreage or 125,000 acres inthe Sacramento Valley by the year 2000, whichever is less. As a result,in 1994-95, about 59% of the rice straw was burned and 38.4% wasdisposed of in the fields by soil incorporation. Off-farm disposal ofrice straw as livestock feed and materials for environmental mitigationand erosion control counted for only 0.6%. Rice growers are underextreme pressure to find alternative environmentally friendly methodsfor straw disposal and/or utilization. If no other practical strawdisposal alternatives are developed to compensate for the burningphasedown, rice farmers will be forced to incorporate an estimated 72.9%of the straw production by the year 2000 to comply with the statutoryrice straw burning phasedown requirements. However, available researchand experience suggest that incorporation rates this high couldpotentially cause reduction in crop yield and increase of foliar diseaseand possible development of adverse soil conditions.

[0014] Rice straw is offered as a single relevant example. The disposalof other solid wastes presents similar problems and new economicaltechnologies for solid waste disposal and/or utilization must bedeveloped. Thus, a method for disposing of agricultural and other wasteswhich utilized an apparatus of simple design, required littleexpenditure of energy to operate and which produced methane as itreduced the volume of disposable solids would represent a significantadvance. Quite surprisingly the present invention provides such methodsand devices.

SUMMARY OF THE INVENTION

[0015] Anaerobic digestion of solid waste, particularly agriculturalwaste, is a promising technique for both generating energy and reducingthe volume of waste which must be disposed of. The energy generated canbe significant. For example, the energy content of a pound of rice strawis about 6,500 Btu (British Thermal Units), and the energy stored in thestraw by growing crop each year in the Sacramento Valley is 1.95×10¹²Btu. Thus, it is realistic to consider agricultural waste as a renewableresource for energy generation.

[0016] Anaerobic digestion is an enhanced biodegradation process thatoffers a promising alternative approach for helping solve problemscaused by agricultural waste such as the imminent rice straw disposalproblems in concentrated rice production regions such as California.Anaerobic digestion uses a consortium of natural bacteria to degrade andthen convert a large portion of solid waste into biogas, which is amixture of methane and carbon dioxide. If captured, biogas can beutilized as a clean fuel for heat and power generation.

[0017] Anaerobic phase digestion (APS) is a new type of two phasesystem. The system employs at least one hydrolysis reactor and abiogasification reactor. In the APS digester system, carbon compounds inthe organic substrates are liquefied into VFAs in the hydrolysisreactor. The soluble VFAs produced are transferred to thebiogasification reactor at a controlled rate. This allows themaintenance of a stable pH level in the biogasification reactor so thatthe optimum growth rate of methanogenic bacteria can be achieved. In afirst aspect, the present invention provides a process for methaneproduction by two-phase anaerobic digestion of organic material. Theprocess comprises incubating a first mixture having a solid organiccomponent and an aqueous liquid component, under anaerobic conditions,in a hydrolysis digester having an upper portion and a lower portion andcontaining a hydrolysis means therein. After a first period ofincubation, a portion of the liquid component of the first mixtureresiding in the lower portion of the hydrolysis digester is transferredto a methane phase digester having an upper portion and a lower portionand a methanogenesis means therein. In the methane phase digester, thefirst mixture is combined with the methanogenesis means to form a secondmixture. The second mixture is incubated for a second period of time,generating methane. The second mixture is intermittently agitated, thenallowed to remain still for a third period of time. After the thirdperiod of time, a portion of the second mixture residing in the upperportion of the methane phase digester is transferred to the hydrolysisphase digester.

[0018] The APS-digester system of the invention has innovative designfeatures that allow it to handle the solid organic substrateseffectively. The hydrolysis reactor is operated in a batch or semi-batchmode to ease the handling of solid materials, and the biogasificationreactor operated continuously to maintain active bacterial culture inthe system and to produce biogas at a relatively constant level. Thedevice used in the system of the invention is of simple design and iseconomical to construct and operate.

[0019] In a second aspect, the present invention provides an anaerobicphased solids digester system for methane production. The systemcomprises a hydrolysis reactor which is separated into upper and lowerportions by a perforated support means. The upper portion of thehydrolysis reactor has a hydrolysis reactor liquid inlet and the lowerportion has a hydrolysis reactor liquid outlet. The device furthercomprises a biogasification reactor. The biogasification reactor has abiogasification reactor gas outlet and, optionally, an agitating means.Similar to the hydrolysis reactor, the biogasification reactor has anupper portion and a lower portion. The upper portion has abiogasification reactor liquid outlet and the lower portion has abiogasification liquid inlet.

[0020] The hydrolysis reactor and the biogasification reactor areconnected via a series of conduits through which liquid from one reactorcan be transferred to another reactor. Thus, the device also comprises afirst conduit connecting the hydrolysis reactor outlet to thebiogasification inlet and a second conduit connecting thebiogasification reactor outlet with the hydrolysis reactor inlet.

[0021] Other features, objects and advantages of the present inventionand its preferred embodiments will become apparent from the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic drawing of the anaerobic solids digestersystem (APS-digester).

[0023]FIG. 2 is a schematic diagram of the laboratory set-up of theAPS-digester system.

[0024]FIG. 3 displays the daily biogas production at differentpretreatment temperatures.

[0025]FIG. 4 displays the accumulative biogas production at differentpretreatment temperatures.

[0026]FIG. 5 displays the pH variation in the hydrolysis reactor duringthe digestion period for different pretreatment temperatures.

[0027]FIG. 6 displays the pH variation in the biogasification reactorduring the digestion period at different pretreatment temperatures.

[0028]FIG. 7 displays the daily biogas production for different physicalpretreatment conditions with thermal pretreatment at 90° C.

[0029]FIG. 8 displays the daily biogas production for differentpretreatment conditions without thermal pretreatment.

[0030]FIG. 9 displays the accumulative biogas production of rice strawfor different physical pretreatment.

[0031]FIG. 10 displays the accumulative biogas production for differentphysical pretreatment without thermal pretreatment.

[0032]FIG. 11 displays the daily biogas production of rice straw withdifferent solids loading rates.

[0033]FIG. 12 displays the accumulative biogas production of rice strawfor different solids loading rates.

[0034]FIG. 13 displays the biogas production of a prototype APS-digestersystem with two hydrolysis reactors and one biogasification reactor fordigestion of rice straw (chopped and 25 mm).

[0035]FIG. 14 is a schematic diagram of the laboratory set-up of: (a)batch; and (b) SEBAC systems.

[0036]FIG. 15 is a schematic diagram of the laboratory set-up of: (a)single batch APS; and (b) multiple batch APS digesters.

[0037]FIG. 16 displays the daily biogas production at different totalsolids (TS) loading levels with the APS digester and batch systems.

[0038]FIG. 17 displays the cumulative biogas production at different TSlading levels with the APS digester and batch systems.

[0039]FIG. 18 displays the pH variation at different TS loading levelswith the APS digester and batch systems.

[0040]FIG. 19 displays the methane content of biogas at different TSloading levels with the APS digester and batch systems.

[0041]FIG. 20 displays the daily and cumulative biogas production at 75g/L TS loading with the APS digester and SEBAC systems.

[0042]FIG. 21 displays the pH variation at 75 g/L TS loading with theAPS digester and SEBAC systems.

[0043]FIG. 22 displays the methane content of the biogas at 75 g/L TSloading with the APS digester and the SEBAC systems.

[0044]FIG. 23 displays the simulated daily biogas production of the APSdigester system with one or twelve hydrolysis reactors.

[0045]FIG. 24 displays the simulated daily biogas production of the APSdigester system with two, three, four, six and eight hydrolysisreactors.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0046] Abbreviations and Definitions

[0047] APS, anaerobic phased solids digester; SEBAC, sequential batchanaerobic composition; TS, total solids, VS, volatile solids; SRT, solidretention time; HRT, hydraulic retention time.

[0048] As used herein, the term “organic substrate” refers tocarbonaceous feedstock which can be used in the process and device ofthe invention to produce methane.

[0049] The terms “biogasification” and “methanogenesis” are used hereinessentially interchangeably

[0050] The present invention provides improved methods for the anaerobicdigestion of waste to produce methane and devices with which to performthese methods.

[0051] Anaerobic phase digestion (APS) is a new type of two phasesystem. The system employs at least one hydrolysis reactor and abiogasification reactor. In the APS digester system, carbon compounds inthe organic substrates are liquefied into VFAs in the hydrolysisreactor. The soluble VFAs produced are transferred to thebiogasification reactor at a controlled rate. This allows themaintenance of a stable pH level in the biogasification reactor so thatthe optimum growth rate of methanogenic bacteria can be achieved. In afirst aspect, the present invention provides a process for methaneproduction by two-phase anaerobic digestion of organic material. Theprocess comprises incubating a first mixture having a solid organiccomponent and an aqueous liquid component, under anaerobic conditions,in a hydrolysis digester having an upper portion and a lower portion andcontaining a hydrolysis means therein. After a first period ofincubation, a portion of the liquid component of the first mixtureresiding in the lower portion of the hydrolysis digester is transferredto a methane phase digester having an upper portion and a lower portionand a methanogenesis means therein. In the methane phase digester, thefirst mixture is combined with the methanogenesis means to form a secondmixture. The second mixture is incubated for a second period of time,generating methane. The second mixture is optionally intermittentlyagitated, then allowed to remain still for a third period of time. Afterthe third period of time, a portion of the second mixture residing inthe upper portion of the methane phase digester is transferred to thehydrolysis phase digester.

[0052] The process of the invention can be practiced with anycarbonaceous organic substrate including, but not limited to, sewagesludge, forestry waste, food waste, agricultural waste, municipal waste,and the like.

[0053] Municipal waste primarily contains cellulosic products,particularly kraft paper. It is known that such cellulosics can bedigested as well as the minor amounts of waste protein, carbohydratesand fat present in municipal waste.

[0054] In a presently preferred embodiment, the organic substrateconsists, at least in part, of an agricultural waste. Agriculturalwastes include both plant and animal wastes. Many types of agriculturalwaste can be used in conjunction with the present invention. Usefulagricultural wastes include, but are not limited to, foliage, straw,husks, fruit, manure and the like.

[0055] The present invention utilizes a separate acid digestion phasewherein fermentation under anaerobic conditions leads to the productionof aldehydes, alcohols and acids. Methane is also generated during thisphase. The methane can be collected directly from the hydrolysis phaseor it can be routed to the biogasification phase for later routing to amethane collection apparatus. The fermentation in the biogasificationphase leads to the production of methane and carbon dioxide. These gasesare collected and they can optionally be passed into a clean up zonewhere the methane and the carbon dioxide are separated. The separatorcan be any separator known to the art which can separate gas componentsprimarily of carbon dioxide and methane,

[0056] Both the hydrolysis phase and the methanogenesis phase areoperative over variable pH ranges that are related to the nature of theorganic substrate and the amount of total solids in the organicsubstrate. In a preferred embodiment, the acid phase pH is maintainedfrom about 4.5 to about 6.5. In another preferred embodiment, thebiogasification phase pH is maintained from about 6.5 to about 7.5.

[0057] Any art known hydrolysis or methanogenesis means can be used inthe present invention. These include, but are not limited to acids,bases, enzymes and combinations of these substances. In a presentlypreferred embodiment, the hydrolysis and methanogenesis means aremicroorganisms.

[0058] Any active hydrolytic or methane producing mesophilic orthermophilic anaerobic digestion system can be used in the presentinvention. Methane-producing anaerobic systems utilizing acid formingbacteria and methane-producing organisms, as are well known to beemployed to produce methane from sewage sludge, can be employed in thepractice of the present invention. A review of the microbiology ofanaerobic digestion is set forth in Anaerobic Digestion, 1. TheMicrobiology of Anaerobic Digestion, D. F. Toerien and W. H. J.Hattingh, Water Research, Vol. 3, pages 385-416, Pergamon Press (1969).As set forth in that review, the principal suitable acid forming speciesinclude, species from genera including, but not limited to, Aerobacter,Aeromonas, Alcaligenes, Bacillus, Bacteroides, Clostridium, Eschericia,Klebsiella, Leptospira, Micrococcus, Neisseria, Paracolobacterium,Proteus, Pseudomonas, Rhodopseudomonas, Sarcina, Serratia, Streptococcusand Streptomyces. Also of use in the present invention aremicroorganisms which are selected from the group consisting ofMethanobacterium omelianskii, Mb. formicium, Mb. sohngenii,Methanosarcina barkerii, Ms. methanica and Mc. mazei and mixturesthereof. Other useful microorganisms and mixtures of microorganisms willbe apparent to those of skill in the art.

[0059] A wide variety of substrates are utilized by the methaneproducing bacteria, but each species is believed to becharacteristically limited to the use of a few compounds. It istherefore believed that several species of methane producing bacteriaare required for complete fermentation of the compounds present incertain organic substrates such as sewage. For example, the completefermentation of valeric acid requires as many as three species ofmethane producing bacteria. Valeric acid is oxidized by Mb. Suboxydansto acetic and propionic acids, which are not attacked further by thisorganism. A second species, such as Mb. Propionicum, can convert thepropionic acid to acetic acid, carbon dioxide and methane. A thirdspecies, such as Methanosarcina methanica, is required to ferment aceticacid.

[0060] An operative mixed culture is capable of sustaining itselfindefinitely as long as a fresh supply of organic materials is addedbecause the major products of the fermentation are gases, which escapefrom the medium leaving little, if any, toxic growth inhibitingproducts. Mixed cultures generally provide the most completefermentation action. Nutritional balance and pH adjustments can be madeas is known in the art to favor hydrolytic activity

[0061] As discussed in U.S. Pat. No., 4,022,665, issued May 10, 1977 toGhosh et al., various studies in the art have demonstrated that a numberof acids are converted to methane and carbon dioxide when such acids arecontacted with mixed anaerobic cultures. For example, the fermentationof acetic, propionic and butyric acids, as well as ethanol and acetone,all result in the production of methane and carbon dioxide. Only theratio of methane to carbon dioxide changes with the oxidation level ofthe particular substrate. Studies in the art have also established thatcarbon dioxide can be methanted by the oxidation of hydrogen. It haseven been suggested that methane fermentation of an acid such, as aceticacid, is a two step oxidation to form carbon dioxide and hydrogenfollowed by a reduction to form methane. The net result is the formationof methane and carbon dioxide. It has also been advanced that carbondioxide could be converted to methane in a step-by-step reductioninvolving formic acid or carbon monoxide, formaldehyde and methanol asintermediates. Whatever the actual underlying mechanism, it is acceptedthat carbon dioxide can participate in the methanation process.Applicants provide the above discussion as useful background and are notbinding themselves to any particular theory of operation.

[0062] Mechanical degradation or chemical treatment of the organicsubstrate may be required either to achieve a particle size appropriatefor use in anaerobic digestion according to the invention or to renderthe carbonaceous components of the organic substrate more accessible tothe digestion media. Suitable methods of mechanical degradation areknown in the art. Various pretreatment of the organic substrate canadvantageously be used with the present invention, such as acid oralkaline hydrolysis.

[0063] The method also contemplates the selective use of predigestionhydrolysis of the organic substrate before introduction into the organicphase, as well as post biogasification hydrolysis of waste removed fromthe biogasification phase. The hydrolysis can be conducted as mild acidor mild alkaline hydrolysis, optionally followed by neutralization ofthe added acid or alkali.

[0064] In a presently preferred embodiment, the organic substrate isrice straw. Previous research has demonstrated the feasibility ofanaerobically digesting a mixture of straw (rice straw and wheat straw)and other agricultural and food wastes, such as animal manure, greenleaves and molasses, using conventional digestion reactors fed inbatches or semicontinuously (Hills, D. J. and D. W. Roberts,Agricultural Wastes 3:179-189 (1981); Dar, G. H. and S. M. Tandon,Biological Wastes 21:75-83 (1987); Adbullah et al., Journal ofAgricultural Sciences 119:255-263 (1992); Somayaji, D. and S. Khanna,World Journal of Microbiology & Biotechnology 10:521-523 (1994)). Theresearch of Hills and Roberts (1981) showed that adding either choppedrice straw or chopped wheat straw to dairy manure enhanced the anaerobicdigestion process and increased the methane production.

[0065] Rice straw is a ligno-cellulosic material mainly composed ofcellulose (37.4%), hemicellulose (44.9%), lignin (4.9%), and silicon ash(13.1%) (Hills, D. J. and D. W. Roberts, Agricultural Wastes 3:179-189(1981)). The straw contains about 0.4% nitrogen and has a carbon tonitrogen ratio (C/N) of around 75. The proper range of C/N ratio foranaerobic digestion is 25-35 (Hills, D. J. and D. W. Roberts,Agricultural Wastes 3:179-189 (1981)). Therefore, nitrogen needs to besupplemented in order to effect the anaerobic digestion of rice straw.Nitrogen can be added in inorganic forms, such as ammonia, or in organicforms such as organic nitrogen contained in urea, animal manure or foodwastes. But once nitrogen is released from the organic matter, it willbecome ammonia (NH₄ ⁺) which is water soluble. Recycling of nitrogen inthe digested liquid will reduce the amount of nitrogen needed forcontinuous operation of anaerobic digesters. Animal manures and foodwastes are good nutrient sources if they are readily available in theareas close to rice straw production. Nitrogen fertilizer, such asammonia or urea, is another source of nitrogen that can be easily addedto the straw and may be more suitable for the areas where handling othertypes of wastes is not feasible.

[0066] Thus, in a preferred embodiment, the organic substrate issupplemented with a nitrogen source. In a further preferred embodiment,the nitrogen source is a member selected from the group consisting ofurea, animal manure, food waste, inorganic nitrogen fertilizers andcombinations thereof.

[0067] Because of its ligno-cellulosic structure, rice straw isdifficult to biodegrade. Its major component, cellulose, is a fibrous,water-insoluble substance. It is a linear, unbranched homopolysaccharideof 10,000 to 15,000 d-glucose units in a crystalline structure(Lehninger, A. L. et al., Principles of Biochemistry (2^(nd) ed.), WorthPublishers, New York, N.Y. (1993)). Another major component,hemi-cellulose, is also water-insoluble and consists of a mixture ofpolymers made up from xylose, arabinose, glucuronic acid and glucose.Breakdown of cellulose and hemi-cellulose through the process ofchemical hydrolysis or biodegradation will release simple sugars andmake them available for further conversion into other products, such asgases in anaerobic digesters. Lignin is a building component for thecell wall of rice straw and forms the barrier around cellulose andhemi-cellulose. It is a complex aromatic polymer of phenylpropanebuilding blocks and is highly resistant to chemical and biologicaldegradation. Lignin is generally considered not biodegradable inanaerobic digesters although it can be degraded by some aerobicmicroorganisms, such as fungi (Hobson and Wheatley, 1992). Thehydrolysis of cellulose can only occur after the lignin structure isdamaged. Enzymes play an important role in biodegradation oflignocellulosic materials. Cellulases, the enzymes that help break downcelluloses, can convert cellulose into glucose with little by-products.However, celluloses cannot easily penetrate through the lignin sealsurrounding cellulose fibers, and therefore, pretreatment of straw, suchas treatment with mechanical grinding and cutting, heat, strong acids oralkaline, are usually helpful.

[0068] Several works have been published on chemical pretreatment ofrice straw to achieve delignification and hydrolysis of cellulose. Thepretreatment methods that have been explored include: bicarbonatetreatment (Liu, J. X. et al., Animal Feed Science and Technology52:131-139 (1994)), radiation (Xin and Kumakura, Bioresource Technology43:13-17 (1992)), alkaline peroxide treatment (Patel and Bhatt, J Chem.Tech. Biotechnol. 53:53-263 (1991)), and ammonia treatment (Sankat andLauckner, Canadian Agricultural Engineering 33(2):309-313 (1991)).

[0069] Thus, in a preferred embodiment, the rice straw is pretreated bya chemical treatment method selected from the group consisting ofbicarbonate treatment, alkaline peroxide treatment, radiation treatment,ammonia treatment and combinations thereof.

[0070] The ammonia treatment shows several advantages over the othertreatment, such as the presence of hydroxyl ions as a delignificationfactor, a source of nitrogen for biodegradation, and no separate wastewater streams generated from the pretreatment process. Thus, in apresently preferred embodiment, the rice straw is treated with aqueousammonia. In a further preferred embodiment, the ammonia is present in anamount of from about 0.5% to about 10%, more preferably from about 1% toabout 5% relative to the total weight of solids derived from rice straw.

[0071] Mechanical size reduction of rice straw will also help with thebiodegradation by rupturing the cell walls and making the biodegradablecomponents more accessible to microorganisms. Thus in a preferredembodiment, the rice straw is pretreated by a physical process selectedfrom the group consisting of grinding, cutting, heating and combinationsthereof. In another preferred embodiment, the rice straw is pretreatedusing a method comprising grinding the rice straw to a size from about 5millimeters to about 50 millimeters. In a further preferred embodiment,the rice straw is heated to a temperature of from about 50° C. to about120° C., more preferably from about 60° C. to about 90° C.

[0072] Portions of the liquid component of the digestion mixture areintermittently exchanged between the hydrolysis digester and thebiogasification digester during the course of the digestion.

[0073] In a preferred embodiment, an amount of liquid from about 10% toabout 50% of a digester's liquid content is exchanged between thedigesters from 1 to 12 times over a 24 hour period, more preferably from4 to 6 times in a 24 hour period.

[0074] In a second aspect, the present invention provides an anaerobicphased solids digester system for methane production. The systemcomprises a hydrolysis reactor which is separated into upper and lowerportions by a perforated support means. The upper portion of thehydrolysis reactor has a hydrolysis reactor liquid inlet and the lowerportion has a hydrolysis reactor liquid outlet. The device furthercomprises a biogasification reactor. The biogasification reactor has abiogasification reactor gas outlet and an agitating means. Similar tothe hydrolysis reactor, the biogasification reactor has an upper portionand a lower portion. The upper portion has a biogasification reactorliquid outlet and the lower portion has a biogasification liquid inlet.

[0075] The hydrolysis reactor and the biogasification reactor areconnected via series of conduits through which liquid from one reactorcan be transferred to another reactor. Thus, the device also comprises afirst conduit connecting the hydrolysis reactor outlet to thebiogasification inlet and a second conduit connecting thebiogasification reactor outlet with the hydrolysis reactor inlet.

[0076] In a preferred embodiment, the system of the invention comprisesadditional hydrolysis reactors. Any number of hydrolysis reactors can beused in conjunction with the present invention. In a preferredembodiment, the system utilizes between 1 and 15 hydrolysis reactors,more preferably between 2 and 8 hydrolysis reactors.

[0077] The hydrolysis reactors and the biogasification reactor can belinked in any useful arrangement selected from parallel linking, serieslinking and combinations thereof. For example, the hydrolysis reactorscan be linked in parallel with the biogasification reactor.Alternatively, the hydrolysis reactors can be linked in series withother hydrolysis reactors and this hydrolysis manifold can be linked tothe biogasification. In still another embodiment, more than one manifoldof hydrolysis reactors can be linked in parallel with thebiogasification reactor.

[0078] Any perforated support means known in the art can be used in thesystem of the invention. The support means can comprise structuresincluding, but not limited to, grids, screen, filters, grates, sieves,slats, strainers and the like. The perforated support means can beconstructed of any material that is substantially inert under thehydrolysis conditions including, but not limited to, plastics, metals,resin, composites, graphite, and the like. Suitable support meansconfigurations and compositions will be apparent tot hose of skill inthe art.

[0079] Any means known in the art for agitating a liquid or suspensioncan be used in the system of the invention. Exemplary means include, butare not limited to, overhead stirrers, gas or motor driven stirrers,magnetic stirrers, shakers, homogenizers, sonicators, gas bubblingtubes, ebulliators and the like. Other useful agitating means will beapparent to those of skill in the art.

[0080] The solids feedstock, such as rice straw, and a bacterial cultureare contained in the hydrolysis reactor. Each hydrolysis reactor workswith semibatches while the biogasification reactor produces biogascontinuously. In preferred embodiment, digesting straw, the straw is fedinto the hydrolysis reactor from the top of the reactor in batches orsemibatches. After the straw is continuously hydrolyzed during eachbatch treatment, the soluble substances produced in the hydrolysisreactor are transferred intermittently to the biogasification reactorfor continuous biogas production. The biogasification reactor contains aconcentrated bacterial. After completing a digestion cycle, the digestedstraw is removed from the hydrolysis reactor before a new batch of strawis added.

[0081] The APS-Digester System has innovative design features that allowit to handle the solid organic substrates effectively. The hydrolysisreactor is operated in a batch or semi-batch mode to ease the handlingof solid materials, and the biogasification reactor operatedcontinuously to maintain active bacterial culture in the system and toproduce biogas at a relatively constant level.

[0082] The operation of the system of the invention will become apparentby reference to FIG. 2. The principle outlined herein with reference tothis figure is equally applicable to those systems utilizing additionalhydrolysis reactors.

[0083] The organic substrate is fed into the hydrolysis reactor 10through an inlet or the top of the vessel 1. The organic material restson top of perforated support 25. The hydrolysis reactor contains atleast sufficient liquid to wet the organic substrate in the hydrolysisreactor. After a period of incubation in the hydrolysis reactor, theliquid containing the hydrolyzed organic substrate is transferred fromthe hydrolysis reactor into the biogasification reactor via firstconduit 40. This transfer process can be assisted by means of a positivedrive pump located inside the hydrolysis reactor, or a negative drivepump located inside the biogasification reactor or along the conduit 40.The mixture in the biogasification reactor is optionally intermittentlyagitated with an agitating means 70. Following a period of incubationand digestion in the biogasification reactor, the liquid containing thedigested organic substrate can optionally be recirculated back into thehydrolysis reactor via second conduit 50. This recirculation can beassisted by a pump as described above, with the caveat that the fluidflow is in the opposite direction, thus, the pumping direction must besimilarly shifted. During the period of incubation in thebiogasification reactor, the digesting organic substrate gives rise to agaseous methane-containing product which is vented through thebiogasification reactor gas outlet 60. When the methane generatingpotential of the organic substrate has been exhausted, the remainingmaterial is removed through an outlet or the top of reactor 10.

[0084] In operation, withdrawing the liquid from the bottom of thehydrolysis reactor prevents disturbing the organic substrate hydrolysisprocess proceeding in the upper portion of the hydrolysis reactor.

[0085] The following examples further define the invention and shouldnot be construed as further limiting. The contents of all references,patents and patent applications cited throughout are expresslyincorporated herein by reference.

[0086] The detailed examples which follow illustrate the device andmethods of the invention as applied to the production of biogas from thedigestion of rice straw.

EXAMPLES

[0087] Example 1 illustrates the method and the device of the inventionin conjunction with the digestion of rice straw.

[0088] Example 2 sets forth a comparative study between the device andthe method of the invention and other art recognized methods ofdigesting organic substrates. Similar to Example 1, Example 2 utilizesrice straw as an exemplary organic substrate.

Example 1

[0089] 1.1 Experimental Materials and Methods

[0090] Two laboratory scale APS-Digester Systems were used for thisstudy. One system had one hydrolysis reactor and one biogasificationreactor as shown in FIG. 2, and the other had two hydrolysis reactorsand one biogasification reactor. All the reactors were made of plexiglaswith inside diameters of 4.5 inches. The total and working volumes ofeach reactor were 5.2 L and 4.0 L, respectively. The rice straw wasprocessed in batches, i.e. the system was operated in a batch mode witha retention time of 24 days. During the digestion, liquid was circulatedintermittently between the hydrolysis reactor and the biogasificationreactor to transport the soluble substances from the hydrolysis reactorto the biogasification reactor. After each batch of treatment, theresidual solids were removed from the hydrolysis reactor and a new batchof rice straw was loaded. All the reactors were heated to a constanttemperature of 35° C. with a circulated and heated water jacket. Thehydrolysis reactor was not mixed while the biogasification reactor wasmixed intermittently (1 minute every 2 hours) by a mechanical mixer.

[0091] Each reactor was connected to a gas collection bag and a wet-tipgas meter which was used to record the daily biogas production volume.Gas samples were taken twice a week from the sampling port on the gascollection line of each reactor and analyzed for the contents of methaneand carbon dioxide using a Gas Chromatograph (GC) equipped with athermal conductivity detector (TCD). The liquid samples were taken fromeach reactor and measured for pH using a pH meter to monitor thestability of the reactors. For each batch digestion, samples of strawbefore and after digestion and samples of the reactor contents beforeand after digestion were taken and analyzed vent for total solids (TS),volatile solids (VS), and pH. The analysis procedures of TS and VSfollowed the standard methods (APHA, 1992). The reductions of TS and VSin the straw after digestion were calculated using the mass balancemethod. The reductions of TS and VS, daily biogas production, and totalbiogas production during the 24 day period were used to evaluate theperformance of the digester system under different operationalconditions. A total of 17 runs were conducted including threerepetitions. All the digestion runs were at a temperature of 35° C. anda retention time of 24 days. The biogasification reactor was initiallyseeded with the sludge taken from an anaerobic digester in the municipalwaste water treatment plant of Davis, Calif.

[0092] To study the changes of elemental components in the rice strawduring the anaerobic digestion, the solid and liquid samples of threebatch treatments were analyzed for various elements including nitrogen(N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), chloride(Cl), magnesium (Mg), silicon (Si), sodium (Na) and carbon (C). Thechemical analysis was conducted by the analytical laboratory of Divisionof Agriculture and Natural Resources (DANR) at the University ofCalifornia at Davis (UC Davis). The changes of the elemental compositionof the straw after the digestion were calculated using the mass balancemethod.

[0093] Effects of different pretreatment methods, including physical(mechanical), thermal, and chemical (ammonia) treatment, on thedigestion of rice straw were investigated. The physical pretreatmentincluded grinding the straw into two lengths (10 mm and 25 mm) with ahammer mill and chopping the straw into one length (25 mm) with acutter. Thermal treatment was conducted by heating the straw in apressure cooker for two hours at three different temperatures (60° C.,90° C. and 110° C.). Tap water was added to the straw in 6 to 1 weightratio prior to the thermal treatment. Chemical treatment was carried outwith 58% ammonia hydroxide solution. Only one ammonia treatment levelwas used for all the digestion runs. The amount of ammonia added to thestraw for each digestion run was 2% based on the dry weight of the strawdigested. This level was determined based on the adjustment of C/N ratioof the treated straw to around 25. This level of ammonia treatment wasalso found to be effective for increasing the digestibility of ricestraw in an in vitro digestibility study of Sankat and Lauckner (1991).A list of digestion runs operated under a combination of differentpretreatment conditions are listed in Table 1. TABLE 1 PretreatmentConditions Chemical Solids Physical Size Thermal Ammonia Loading Run ofStraw Temperature (%)* Rate (g/L) 1 25 mm (ground) no treatment 2 50 225 mm (ground) 60° C. 2 50 3 25 mm (ground) 90° C. 2 50 4 25 mm (ground)110° C.  2 50 5 10 mm (ground) 90° C. 2 50 6 25 mm (ground) 90° C. 2 507 25 mm (chopped) 90° C. 2 50 8 whole 90° C. 2 50 9 25 mm (ground) notreatment 2 50 10 25 mm (chopped) no treatment 2 50 11 whole notreatment 2 50 12 25 mm (chopped) no treatment 2 50 13 25 mm (chopped)no treatment 2 75 14 25 mm (chopped) no treatment 2 100 

[0094] Three digestion runs (3, 10 and 11) were repeated to validate thetesting procedures used in this study. After finding the differencebetween the repetitions was less 5%, all the other digestion runs werecarried out as a single run for each pretreatment condition in order tosave the time for laboratory operations.

[0095] 1.2 Results

[0096] 1.2a Characteristics of Rice Straw

[0097] Rice straw was collected in bales from a county in northernCalifornia and transported to the laboratory. The characteristics of rawrice straw are presented in Table 2. TABLE 2 C N P K H S TS VS Ash (%)(%) (%) (%) (%) (%) (%) (%) (%) 34.80 0.46 0.09 1.58 4.61 0.14 92.1279.50 20.50

[0098] 1. 2b Effects of Thermal Pretreatment

[0099] The temperature used for pretreatment did have a significanteffect on the digestibility of rice straw as shown in Table 3 withregards to solids (TS and VS) reduction and biogas production. A highertemperature resulted in higher conversion rates of solids and higherbiogas production. As compared with non-pretreatment, the TS and VSreductions were increased by 3.4 22.4% and 3.6-22.6%, respectively, andthe biogas yield increased by 2.5-17.5% when pretreatment temperaturevaried from 60° C. to 110° C. The temperature effect was not linear,however. The increase of solids reduction (15.6%) and biogas productionwas more significant when the temperature increased from 60° C. to 90°C.

[0100] The positive temperature effect may be explained by the increasedchemical reaction rate between the components of rice straw and ammoniahydroxide which was added prior to heating. More soluble compounds werereleased from the straw during the thermal treatment process at highertemperatures and made available to subsequent bacterial degradation.This is clearly reflected by the daily biogas production data as shownin FIG. 3. A higher pretreatment temperature resulted in higher dailybiogas production rate during the first six days of digestion. FIG. 4shows the accumulative biogas production for different pretreatmenttemperatures. Higher pretreatment temperatures resulted in more acidproduction and lower pH levels initially in the hydrolysis reactor asshown in FIG. 5. The initial pH was below 6.0 for 90° C. and 110° C.pretreatment. As digestion progressed and acids were transported to andconsumed in the biogasification reactor, the pH of the hydrolysisreactor was slowly increased to the neutral level (around 7.0). The pHlevel of the biogasification reactor for all pretreatment temperatureswas maintained relatively constant throughout the digestion as shown inFIG. 6. Therefore, the biogasification reactor provided both chemicaland biological buffering capacities for the hydrolysis reactor, makingthe digester system stable in operation. TABLE 3 Methane TS VS ContentPretreatment Reduc- Reduc- Total Biogas Biogas of Temperature tion tionProduction Yield] Biogas (° C.) (%) (%) (L) (L/gVS fed) (%) No treatment40.6 48.4 63.5 0.40 49.4  60° C. 44.0 52.0 65.3 0.41 49.9  90° C. 59.667.6 74.2 0.46 51.4 110° C. 63.0 71.0 75.4 0.47 52.1

[0101] From the biogas production data as shown in FIGS. 3 and 4, we cansee that the digestion process slowed down after two weeks when thehydrolysis of straw and release of soluble sugars became the limitingstep. About 75-80% of the biogas was produced in the first two weeks.This implies that if the retention time for a digestion system isdesigned to be 14 days instead of 24 days, the digester size can bereduced by 42% for a sacrifice of 21-25% biogas production.

[0102] 1.2c Effects of Physical Pretreatment

[0103] Table 4 shows the effects of size reduction of rice straw bymechanical processing (grinding or chopping) on the solids reduction andbiogas production. Generally speaking, the smaller the straw particleswere, the better the digestion was, i.e. the more solids reduction wasand the higher the biogas yield was. Grinding yielded the best digestionresults, because milling broke up the cell walls of straw better thanchopping alone and made the inside of the straw more accessible forchemical and biological breakdown. Such effects of size reduction areclearly shown in the daily biogas production data (FIG. 7). More solublesugars were available in the reactors during the initial nine days,yielding a higher biogas production rate, if the straw was processedinto smaller particles.

[0104] Size reduction appears to have more significant effects whencombined with thermal pretreatment than without thermal pretreatment.The biogas yield of ground, 10 mm, thermally pretreated straw was 0.47L/g VS fed, which is 17.5% higher than the yield of thermally pretreatedwhole straw (0.40 L/gVS fed). If comparing the ground straw with thechopped straw, we noticed that the biogas yield of ground, 25 mm,thermally pretreated straw was 0.46 L/g VS fed, 12.2% higher thanchopped, 25 mm, thermally pretreated straw.

[0105] The chopped straw was very close to the whole straw for thedigestion, showing only 2.5% increase in the biogas yield. Improvementof digester performance by mechanical processing (milling and chopping)was very small if the straw was not thermally pretreated.

[0106] The digestion rates of these three straws were very close asshown in FIG. 8. FIGS. 9 and 10 show the accumulative biogas productionof rice straw for different physical pretreatment conditions with andwithout thermal pretreatment. TABLE 4 Thermal Biogas Methane PhysicalPretreat- TS VS Yield Content Pretreatment ment Reduction Reduction(L/gVS of Biogas (Size of straw) (° C.) (%) (%) fed) (%) 10 mm 62.4 69.60.47 51.1 (ground) 25 mm 90 59.6 67.6 0.46 50.6 (ground) 25 mm 44.8 60.00.41 50.1 (chopped) Whole 43.0 56.4 0.40 50.0 25 mm 40.6 48.4 0.40 49.4(ground) 25 mm None 37.3 43.8 0.38 50.0 (chopped) Whole 36.3 42.4 0.3850.5

[0107] 1.2d Effects of Total Solids Loading Rate

[0108] All the results reported as above were obtained from thedigestion runs with the same total solids (TS) loading rate in thehydrolysis reactor of 50 g/L. Potential of increasing the solids loadingrate was investigated with three levels of TS loading rate, 50 g/L,75g/L and 100 g/L, with the chopped, 25 mm, not thermally pretreatedstraw. A higher solids loading rate means a smaller digester system fortreating a given amount of rice straw.

[0109] Table 5 shows the solids reduction and biogas production forthree solids loading rates. The digester system performed better at ahigher loading rate. When the loading rate was increased from 50 g/L to100 g/L, the solids reduction and biogas yield increased by about 10%.The initial concentration of bacterial mass as measured by the mixedliquor volatile suspended solids (MLVSS) in the biogasification reactorwas controlled at 1.2% for all the three loading rates. A higher biogasyield at a higher loading rate means that the capacity of bacteria inthe reactors was better utilized at a higher loading rate. Futureresearch will study the optimum food to microorganism ratio (F/M) in thesystem for different solids loading rates. TABLE 5 Methane TS LoadingContent of Rate TS Reduction VS Reduction Biogas Yield Biogas (g/L) (%)(%) (L/g VS fed) (%) 50 35.8 43.8 0.38 50.0 75 37.3 44.9 0.39 49.4 100 40.1 48.4 0.42 50.5

[0110] 1.2e Changes of Elemental Composition of Rice Straw DuringAnaerobic Digestion

[0111] Table 6 lists the contents of elemental components in rice strawbefore and after digestion as obtained from the digestion run withground, 25 mm straw thermally pretreated at 60° C. The contents of bothN and P in the rice straw increased after the digestion. The N contentof straw residue from the digester was twice as much as the N content ofraw straw. This is beneficial for use of such straw residues as soilamendment because of increased nutrient contents and reduced carboncontents as compared with raw rice straw. The contents of all the otherelements as listed in the table decreased after the digestion. Thecontents of K, Cl, and S were reduced by 90%, 87%, and 43%,respectively. These three elements, together with silicon (Si) are themajor problematic elements for combustion of rice straw, causingslagging and fouling of the boilers (Jenkins, B. M. et al. Biomass andBioenergy 10(4):177-200 (1995)). Reduction of these elements throughanaerobic digestion will make the straw residues a more desirablebiomass fuel for combustion. Preliminary results of combustion testswith the straw residue showed that the residue was combustedsuccessfully without causing fouling problems even when the combustiontemperature reached 1600° C., as compared to the fact that raw ricestraw usually starts to cause the fouling problems at 1400° C. (Jenkins,B. M., Net Energy Analysis, EBS 216 Class Handout, University ofCalifornia at Davis (1997)). The average heating value of the residuestested was 14.44 MJ/Kg, as compared to 14.75 MJ/Kg for raw rice straw.TABLE 6 Rice N P K Ca Cl Mg S Na C straw (%) (%) (%) (%) (%) (%) (%) (%)(%) Before 0.457 0.09 1.58 0.24 0.87 0.21 0.028 0.023 34.8 diges- tionAfter 0.947 0.11 0.16 0.28 0.11 0.14 0.016 0.017 32.7 diges- tion

[0112] 1.2f Operation of APS-Digester System for Continuous BiogasProduction

[0113] Batch digestion tests reported as above have shown thefeasibility of using the APS-Digester for biogasification of rice strawwith a supplement of nitrogen source, such as ammonia. Batch digestionis featured with cyclic biogas production. In practical applications,the APS-Digester system may be designed to use more than one hydrolysisreactor to couple with the biogasification reactor so that the space ofbiogasification reactor can be utilized more efficiently and the biogasproduction can be maintained at a relatively constant level, which isnormally required by the operation of an engine-generator system forelectrical power generation. From the daily biogas production data ofbatch digestion runs (FIGS. 3, 7, 8 and 11), we can see that thedigestion rate of each batch of rice straw was slowed down after about14 days. Introducing a new batch of feedstock at this time will sustainthe biological activities in the digestion system, especially in thebiogasification reactor, so to keep the biogas production continuouslyat a high level. FIG. 13 shows the biogas production of a prototypeAPS-Digester System with two hydrolysis reactors and one biogasificationreactor. The system was operated for 47 days and three batches of ricestraw was digested. The straw was chopped and 25 mm long without thermalpretreatment. We can see that cyclic variation of biogas production wasmuch damped and the biogas production was continuous. With proper designof the operational schemes in terms of feedstock loading and unloadingand retention time, the APS-Digester system will become a viable andhighly efficient anaerobic digestion system for biogasification ofbiomass materials such as rice straw.

Example 2

[0114]2.1 Materials and Methods

[0115] 2.a General

[0116] Two sets of experiments were designed to compare Batch systemwith the APS-Digester system using single-batch digestion and to compareSEBAC system with the APS-Digester system using multiple-batchdigestion, respectively. The schematic diagrams of three digestionsystems used are shown in FIGS. 14 and 15. The engineering features andoperational procedures of individual systems are described as follows.

[0117] 2.1b Description and Operation of, Anaerobic Digestion Systems

[0118] All the anaerobic reactors used were made of plexiglass and had atotal and working volume of 5.2 and 4.0 L each, respectively. All thereactors were maintained at 35±1° C. using heated circulating waterjackets. Each reactor was connected to a gas collection bag and awet-tip gas meter, which measured the biogas production (L) per day.Ammonia hydroxide solution (58%) was added to rice straw for all thedigestion runs to adjust the C/N of rice straw 25 prior to digestion.Duplicative tests were performed for all the digestion runs. The datareported in this paper are the average of duplicate test runs.

[0119] The batch system was operated as a single-batch digestion system.Three different TS loading levels of 50g/L. 75 g/L. and 100 g/L weretested. The corresponding amount of dry straw used 200 g. 300 g. and 400g. respectively. The TS loading was defined as the amount of dry ricestraw (g) loaded per unit working volume (L) of hydrolysis reactor. Foreach batch digestion, rice straw (chopped into 1-inch length) was mixedwith anaerobic seed sludge collected from a mesophilic digester as themunicipal wastewater plant in Davis, Calif. The amount of the seedsludge used was determined to provide biomass equal to 40% of volatilesolids (VS) in 200 g of rice straw on a dry weight basis. Water wasadded to the reactor to achieve the final TS concentrations of 5%, 7.5%,and 10% for 50 g/L, 75 g/L, and 100 g/L TS loading levels, respectively.Each batch digestion proceeded for 24 days.

[0120] The SEBAC system (FIG. 14-b) was operated as a multiple-batchdigestion system. Both reactors in the system are solid-bed reactorswith a perforated steel plate placed in the lower part of each reactorto allow liquid collection at the bottom of the reactor. The first batchdigestion was started with the mixture of rice straw and anaerobic seedsludge in the same way as with the Batch system described above. After12 days, when the digestion process in the first batch was established,the second batch digester was started with the mixture of rice straw andwater which had a TS concentration of 7.5%. Intermittent liquidcirculation (one minute for every two hours) between two reactors at aconstant flow rate of 600 mL/min was initiated as soon as the secondbatch was started to allow the inoculum to transfer. After 24 days, thefirst batch digestion was finished. The residual solids were taken outand the third batch was carried out in the same way as the second batch.A total of three batches were monitored using a digestion period of 42days. The second and third batches of digestion were assumed torepresent typical operation of a SEBAC system. The laboratory set up ofthe SEBAC system is presented in FIG. 14-b.

[0121] Two types of the APS-Digester system were used. The first system(FIG. 15-a) had one hydrolysis reactor and one biogasification reactor.The first system was used to compare with the Batch system and thereforeoperated as a single-batch digestion system. The second system was usedto compare with the SEBAS system and operated as a multiple-batchdigestion system. A perforated steel place was placed in the lower partof each hydrolysis reactor to allow the liquid collection. With thefirst system, the biogasification reactor was initially seeded withanaerobic seed sludge to provide the Mixed Liquid Volatile SuspendedSolid (MLVSS) of 11,000 mg/L. The hydrolysis reactor was started withrice straw and water. Liquid was recirculated between the two reactorsonce every two hours at a constant flow rate of 600 mL/min. Right afterrecirculation, the biogasification reactor was mixed for one minute andthen allowed to react quiescently with biomass settled to the bottomprior to next recirculation. Three TS loading levels of 50 g/L, 75 g/Land 100 g/L were tested. The second system—the multiple-batchAPS-Digester system was started in the same way as the single-batchAPS-Digester system described above. After 12 days of operation with thefirst batch system, the second hydrolysis reactor loaded with rice strawand water was put in line. The liquid recirculation and reactor mixingsequence between the second hydrolysis reactor and the biogasificationreactor was the same as in the first system but with one-hour delay. ATS loading level of 75 g/L was used in the second system to compare withthe SEBAC system. The system operation was monitored for the same lengthof time (42 days) as with the SEBAC system.

[0122] Finally, computer simulation was performed for a modelAPS-Digester system with a capacity of processing one ton of dry strawper day to study the variation of daily biogas production as affected bythe number of hydrolysis reactors. One biogasification reactor wascoupled with different numbers of hydrolysis reactors (one, two, three,four, six, eight, and twelve). The daily biogas production data from thelaboratory test with the TS loading level of 100 g/L were used in thesimulation. Each system was stimulated for a period of four months witha retention time of 24 days for each batch digestion. For eachsimulation, hydrolysis reactors were started in sequence. For example,for the system with one biogasification reactor coupled witheight-hydrolysis reactors, the batch digestion in the hydrolysisreactors was three days apart in schedule. The daily biogas production(L/day) was calculated for each simulation.

[0123] 2.1c Analytical Procedure

[0124] Gas samples were taken daily from the sampling port in the gascollection line of each reactor and analyzed for the contents of methane(CH₄ and carbon dioxide (CO₂) using a Gas Chromatography (GC) equippedwith a thermal conductivity detector (TCD). The liquid samples weretaken from each reactor and measured for pH using a pH meter. Before andafter the digestion, both liquid and solid samples from each reactorwere taken to analyze for TS and VS concentrations. The reductions of TSand VS for each treatment were calculated based on mass balances. Theanalysis procedures of TS and VS followed the standard methods (APHA,1992).

[0125] 2.2 Results

[0126] 2.2a General

[0127] The rice straw used in this study was collected in bales innorthern California and transported to the laboratory. Thecharacteristics of the rice straw as determined from three replicatesare presented in Table 7. The C/N of rice straw was 76. Ammonia wastherefore added to adjust the C/N to 25, which was found to be theoptimum level for anaerobic digestion (Hills and Roberts, 1981**). TABLE7 C N P K H S TS VS Ash (%) (%) (%) (%) (%) (%) (%) (%) (%) 34.81 0.460.09 1.58 4.61 0.14 92.12 79.50 20.50 ±0.44 ±0.021 ±0.008 ±0.024 ±0.05±0.01 ±0.89 ±0.45 ±0.21

[0128] 2.2b Comparison of the APS-Digester with the Batch System

[0129] The daily and cumulative biogas production, methane contents ofbiogas, and pH variation for both APS-Digester and batch systems arepresented in FIGS. 16-19. The average methane yield, methane content ofbiogas, and reductions of TS and VS are presented in Table 8. Themethane yield with the APS-Digester system increased from 0.38 to 0.42L/g VS added with the TS loading was increased from 50 to 100 g/Lwhereas the methane yield with the batch system decreased from 0.37 to0.05 L/g VS added. The increase of the methane yield with theAPS-Digester system could be explained by the ability of methanogenicbacteria in the biogasification reactor to handle a higher organicloading level. The decease of the methane yield with the batch systemmight be caused by the excessive accumulation of VFSs, leading to therapid drop of pH to a level (below 6.0) that became inhibitory to themethanogenic bacteria. Therefore, the APS-Digester system showedadvantages over the batch system by having higher TS and VS reductions,higher methane content of the biogas and smaller variation of pH duringdigestion.

[0130] With the batch system, it should also be noticed that the dailybiogas production rapidly increased shortly after the digestion wasinitiated and reached to the maximum on the fourth day for all three TSloading levels. The biogas produced during the first four days wasessentially carbon dioxide (CO₂). This indicates that soluble sugarswere released quickly during these initial period. The acetogenicbacteria were responsible for the acid and CO₂ production and pHdecrease. When the TS loading became too high, such as at 75 g/L,accumulation of VFAs in the system lead to inhibition of methanogenicbacteria, resulting in reduced or stopped biogas production. The resultsshowed that the TS loading in the batch system should be limited to 50g/L. In contrast, the APS-Digester did not show VFA inhibition even atthe highest loading level (100 g/L) tested. This was reflected by thedaily and cumulative biogas production as shown in FIGS. 16 and 17. Athigher loading levels (75 g/L and 100 g/L), the daily biogas productionin the APS-Digester system was much higher during the first several daysthan in the batch system. The methane content of the biogas was alsomuch higher. TABLE 8 Digester System APS Batch Total Solid Loading (g/L)50 75 100 50 75 100 Methane Yield (L/g VS 0.38 0.38 0.42 0.37 0.19 0.05added) Methane Content in Biogas 50.10 49.14 50.60 41.45 37.63 27.72 (%)Total Solid Reduction (%) 37.48 36.59 40.67 35.88 16.56 5.33 VolatileSolids Reduction (%) 43.18 44.28 49.14 47.66 22.51 8.01

[0131] 2.2c Comparison of the APS-Digester with SEBAC Systems

[0132] The daily and cumulative biogas production, methane content ofbiogas, and pH variation during digestion are presented in FIGS. 20-22.The pH of the APS-Digester system was measured to be the pH in thebiogasification reactor. The average methane yield, methane content, andreductions of TS and VS are presented in Table 9. The two systemsachieved similar methane yield and reductions of TS and VS. However, thebiogas produced from the APS-Digester system had higher methane content(50.22% on average) than the biogas from the SEBAC biogas productionthroughout digestion. This is because the biogasification reactor in theAPS-Digester system provided buffering capacity for the system andbetter environmental conditions to methanogenic bacteria, resulting inhigher methane production. Therefore the APS-Digester system is found tobe more advantageous than the SEBAC system in terms of methaneproduction and process stability. TABLE 9 Digester Performance APS SEBACTotal Solid Loading (g/L) 75.00 75.0 Methane Yield (L/g VS added) 0.340.35 Methane Content (%) 50.22 40.78 Total Solid Reduction (%) 35.6636.21 Volatile Solids Reduction (%) 40.68 41.08

[0133] 2.2d Computer Simulation of the APS-Digester System for BestDesign Configurations

[0134] Computer simulation was conducted to analyze the biogasproduction profile of the APS-Digester system with one biogasificationreactor coupled with different numbers of hydrolysis reactors. Thepredicted daily biogas production with one biogasification reactorcoupled with one, two, three, four, six, eight, and twelve hydrolysisreactors are presented in FIGS. 23-24 and the predicted average dailybiogas production and its variance are presented in Table 10. Thevariation of daily biogas production became smaller with the increase ofthe number of hydrolysis reactors. With the processing capacity of 1ton/day, the daily biogas production was 365 m³/day for all thecombinations after a start-up period of 24 days. However, the variationsof daily biogas production decreased from 17.77% to 4.05% and 0.14% whenthe numbers of hydrolysis reactors were increased from two to eight andtwelve, respectively. The least variation in daily biogas production wasachieved with one biogasification reactor coupled with twelve hydrolysisreactors. TABLE 10 Num. of Hydrolysis Reactors 1 2 3 4 6 8 12 Ave. DailyBiogas 365.00 365.00 365.00 365.00 365.00 365.00 365.00 (m³/day)Standard Deviation (m³) ±179.82 ±64.86 ±38.23 ±24.06 ±20.09 ±14.78 ±0.50Std. Dev./Average (%) 49.27 17.77 10.47 6.59 5.50 4.05 0.14

[0135] It is to be understood that the above description and is intendedto be illustrative and not restrictive. Many embodiments will beapparent to those of skill in the art upon reading the abovedescription. The scope of the invention should, therefore, be determinednot with reference to the above description, but should instead bedetermined with reference to the appended claims, along with the fullscope of equivalents to which the claims are entitled. The disclosuresof all articles and references, including patent applications andpublications are incorporated herein by reference

What is claimed is:
 1. A method for methane production by two-phaseanaerobic digestion of solid organic material, said method comprising:(a) incubating for a first period of incubation, a first mixturecomprising said solid organic material and an aqueous liquid, underanaerobic conditions, in a first hydrolysis digester having an upperportion and a lower portion and containing a hydrolysis means therein;(b) after said first period of incubation, transferring a portion ofsaid aqueous liquid of said first mixture residing in said lower portionof said hydrolysis reactor to a methane phase digester to form a secondmixture, said methane phase digester having an upper portion, a lowerportion and a methanogenesis; (c) incubating said second mixture for asecond incubation period during which methane is generated; (d)transferring a portion of said second mixture residing in said upperportion of said methane phase digester to said first hydrolysis phasedigester for a third incubation period.
 2. The method according to claim1, further comprising intermittently agitating said second mixture. 3.The method according to claim 1, wherein said solid organic material isa member selected from the group consisting of sewage sludge, forestrywaste, food waste, agricultural waste, municipal waste and combinationsthereof.
 4. The method according to claim 1, wherein said solid organicmaterial comprises agricultural waste.
 5. The method according to claim4, wherein said agricultural waste comprise rice straw.
 6. The methodaccording to claim 1, further comprising collecting said methanegenerated in steps (c) through (e).
 7. The method according to claim 1,wherein methane is generated in step (a).
 8. The method according toclaim 7, further comprising collecting said methane generated in step(a).
 9. The method according to claim 1, wherein said first mixture hasa pH of from about 4.5 to about 6.5.
 10. The method according to claim1, wherein said second mixture has a pH of from about 6.5 to about 7.5.11. The method according to claim 1, wherein said hydrolysis meanscomprises a bacterial culture.
 12. The method according to claim 11,wherein said bacterial culture is a member selected from the groupconsisting of Aerobacter, Aeromonas, Alcaligenes, Bacillus, Bacteroides,Clostridium, Eschericia, Klebsiella, Leptospira, Micrococcus, Neisseria,Paracolobacterium, Proteus, Pseudomonas, Rhodopseudomonas, Sarcina,Serratia, Streptococcus and Streptomyces, Methanobacterium omelianskii,Mb. formicium, Mb. sohngenii, Methanosarcina barkerii, Ms. methanica andMc. mazei and mixtures thereof.
 13. The method according to claim 1,wherein said methanogenesis means comprises a bacterial culture.
 14. Themethod according to claim 11, wherein said bacterial culture is a memberselected from the group consisting of Aerobacter, Aeromonas,Alcaligenes, Bacillus, Bacteroides, Clostridium, Eschericia, Klebsiella,Leptospira, Micrococcus, Neisseria, Paracolobacterium, Proteus,Pseudomonas, Rhodopseudomonas, Sarcina, Serratia, Streptococcus andStreptomyces, Methanobacterium omelianskii, Mb. formicium, Mb.sohngenii, Methanosarcina barkerii, Ms. methanica and Mc. mazei andmixtures thereof.
 15. The method according to claim 2, wherein saidagitating is carried out for between 30 seconds and 10 minutes everyhour.
 16. The method according to claim 1, further comprisingpretreating said solid organic material prior to said first period ofincubation by a method which is a member selected from the groupconsisting of chemical pretreatment, mechanical pretreatment, heatpretreatment and combinations thereof.
 17. The method according to claim16, wherein said pretreating is mechanical pretreatment which is amember selected from the group consisting of cutting, grinding andcombinations thereof.
 18. The method according to claim 16, wherein saidpretreating is chemical pretreatment which is a member selected from thegroup consisting of bicarbonate treatment, radiation treatment, alkalineperoxide treatment, ammonia treatment and combinations thereof.
 19. Themethod according to claim 16, wherein said pretreating is heatpretreatment at a temperature from about 50° C. to about 120° C.
 20. Themethod according to claim 19, wherein said temperature is from about 60°C. to about 90° C.
 21. The method according to claim 16, wherein saidsolid organic material is rice straw and said pretreating comprises: (a)grinding said rice straw; (b) heating said rice straw; and (c) treatingsaid rice straw with ammonia.
 22. The method according to claim 19,wherein said grinding produces rice straw sized from about 5 millimetersto about 50 millimeters.
 23. The method according to claim 22, whereinsaid heating is at a temperature of from about 50° C. to about 120° C.24. The method according to claim 23, wherein said heating is at atemperature of from about 60° C. to about 90° C.
 25. The methodaccording to claim 22, wherein said treating with ammonia utilizes anamount of ammonia equal to about 0.5% to about 10% of the total weightof the rice straw.
 26. The method according to claim 25, wherein saidamount of ammonia is equal to about 1% to about 5% of the total weightof the rice straw.
 27. The method according to claim 1, wherein a memberselected from the group consisting of said first incubation period, saidfourth incubation period and combinations thereof occur in a hydrolysisreactor which is not said first hydrolysis reactor.
 28. An anaerobicphased solids digester system for methane production, said systemcomprising: a first hydrolysis reactor containing therein a perforatedsupport means separating the reactor into an upper portion and a lowerportion, the upper portion having a hydrolysis reactor liquid inlet andthe lower portion having a hydrolysis reactor liquid outlet; abiogasification reactor having a biogasification reactor gas outlet, anagitating means, an upper portion and a lower portion, the upper portionhaving a biogasification reactor liquid outlet and the lower portionhaving a biogasification liquid inlet; a first conduit connecting thehydrolysis reactor outlet to the biogasification inlet; and a secondconduit connecting the biogasification reactor outlet with thehydrolysis reactor inlet.
 29. The digester system according to claim 28,further comprising between 1 and 15 additional hydrolysis reactors. 30.The digester system according to claim 29, wherein said hydrolysisreactors and said methanogenesis reactor are linked in a manner selectedfrom the group consisting of parallel linking, series linking andcombinations thereof.
 31. The digester system according to claim 29,wherein said hydrolysis reactors are linked in parallel with saidmethanogenesis reactor.
 32. The digester system according to claim 29,wherein said hydrolysis reactors are linked in series with saidmethanogenesis reactor.
 33. The digester system according to claim 29,wherein said perforated support means is a member selected from thegroup consisting of grids, filters, grates, sieves, slats, strainers andcombinations thereof.
 34. The digester system according to claim 27further comprising a pump operably connected to said first hydrolysisreactor.